The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of a rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Sensors can also be categorized as current in plane (CIP) sensors or as current perpendicular to plane (CPP) sensors. In a CIP sensor, current flows from one side of the sensor to the other side parallel to the planes of the materials making up the sensor. Conversely, in a CPP sensor the sense current flows from the top of the sensor to the bottom of the sensor perpendicular to the plane of the layers of material making up the sensor. In a CPP sensor design, the magnetic shields usually double as electrical leads for supplying a sense current to the sensor. Therefore, in CPP sensor design, the shields/leads contact the top and bottom of the sensor.
The ever increasing demand for data storage density and data rate has increasingly pushed the limits of data storage designs. Recently in efforts to overcome such limits, engineers and scientists have focused on the use of perpendicular recording. In a perpendicular recording system a write pole emits a highly concentrated magnetic field that is directed perpendicular to the surface of the medium (eg. the disk). This field in turn magnetizes a localized portion of the disk in a direction perpendicular to the surface of the disk, thereby creating a bit of data. The resulting flux travels through the disk to a return path having a much larger area than the area in which the bit was recorded. The increased interest in perpendicular recording has lead to an increased interest in current perpendicular to plane (CPP) sensors, which are particularly suited to use in perpendicular recording. This is in part because of the ability of CPP GMR sensors to have smaller gap thicknesses, and therefore smaller bit lengths.
Another effort to increase data capacity and data rate has involved the development of self-pinned sensors. As discussed above, sensors have been constructed as AP pinned sensors having first and second magnetic layers (AP1 and AP2) that are antiparallel coupled across a coupling layer such as Ru. The magnetic layer furthest from the free layer (AP1) is then exchange coupled with a layer of antiferromagnetic material (AFM) layer, which strongly pins the moment of that layer. However, to be useful, such AFM layers must be very thick, even being as thick or thicker than all of the other sensor layer combined. In an effort to decrease gap height, thereby decreasing bit length, sensors have recently been developed that have antiparallel coupled pinned layers that can maintain their pinning with out the need for an AFM layer. Such sensors take advantage of the strong positive magnetostriction of certain material, which when combined with compressive stresses in the sensor, causes a strong magnetic anisotropy in a desired direction perpendicular to the ABS.
Whether a sensor uses a conventional AFM pinned AP coupled pinned layer or a self pinned AP coupled pinned layer, a problem remains in that the outer most magnetic layer (pinning layer or AP1) detracts from the performance of the sensor. The magnetic layer closest to the free layer (pinned layer or AP2) contributes positively, and strongly to the magnetoresistive performance of the GMR sensor, based on spin dependent scattering as discussed above. However, since the pinning layer, that which is furthest from the free layer, is oriented 180 degrees out of phase with the pinned layer, its contribution to GMR is opposite to that of the pinned layer and is, therefore, subtractive. Since the pinning layer is further from the free layer and spacer layer than the pinned layer is, its subtractive GMR effect is less than the positive GMR effect of the pinned layer so there is a net positive GMR effect defined by the orientation of the pinned layer. However, the subtractive GMR effect of the pinning layer can be as great as 30% to 50%.
Another factor affecting sensor performance is the thickness of sensor layers. It has been known that the GMR performance (dR) increases with increasing pinned layer thickness. However, as the thickness of the magnetic layers (pinned layer and pinning layer) increases, the pinning strength drops off significantly, resulting in pinned layer instability. Therefore, while increased pinned layer thickness would be desirable to maximize the dR performance of the sensor, such an increase in thickness is limited by the need to maintain sufficient pinned layer pinning.
Therefore there is a strong felt need for a GMR structure that will minimize the subtractive GMR of the AP1 layer (pinning layer) in an AP pinned layer structure. There is also a need for a sensor structure that can take advantage of the dR performance increase realized by increased pinned layer thickness, while still maintaining pinned layer stability. Such a structure would preferably be useful in a CPP structure since that is this structure is most promising for use in future perpendicular recording systems. Such a structure would also preferably be advantageous for use in a self pinned structure, in order to take advantage of the gap thickness reduction provided by such self pinned structures.